The present invention relates to a wavelength division multiplexed (WDM) cross-connect device for use in optical networks. More particularly, the present invention relates to a WDM cross-connect device that may be configured to be strictly non-blocking.
A wavelength division multiplexed (WDM) cross-connect device, hereinafter referred to as a WDM cross-connect, is a network of fibers connected to various optical components that allow a set of input fibers to be connected to a set of output fibers. Each fiber in the network can support some number n of wavelength channels. In other words, at any time there can be up to n signals along a fiber with each signal using a distinct wavelength. WDM cross-connects are capable of performing wavelength interchanging by connecting a wavelength channel on an input fiber to a different wavelength channel on an output fiber. WDM cross-connects comprise components that are capable of switching an incoming wavelength channel onto any different wavelength channel on an outgoing fiber. These components are commonly referred to as wavelength interchangers.
Another type of component comprised by WDM cross-connects is known as an optical switch. An optical switch has an arbitrary number of fibers passing into it and out of it and any wavelength channel on any incoming fiber can be switched to the same wavelength channel on any outgoing fiber, assuming the wavelength channel is not already being used. The WDM cross-connect also comprises optical fibers that are connected to the optical switches and to the wavelength interchangers at nodes. The optical fibers provide directed paths through the WDM cross-connect in the sense that the signal on any optical fiber only travels in a forward direction through the cross-connect and can never meet itself.
When a request for a connection in a WDM cross-connect is made, the WDM cross-connect must perform two fundamental tasks. First of all, a route or path must be found in the WDM cross-connect from the requested input fiber to the requested output fiber. Secondly, for each fiber in the route, an unused wavelength channel must be assigned so that (1) the wavelength channels assigned on the input and output fibers are the requested wavelength channels, and (2) the wavelength channels assigned on any two consecutive fibers in the route are the same, unless there is a wavelength interchanger connecting the two consecutive fibers.
WDM cross-connects have been proposed that have xe2x80x9cnon-blockingxe2x80x9d properties. The term xe2x80x9cnon-blockingxe2x80x9d corresponds to the ability of the WDM cross-connect to satisfy requests for connections, i.e., the requests are not xe2x80x9cblockedxe2x80x9d as a result of an unavailable route or wavelength channel. Some of these WDM cross-connects are rearrangeably non-blocking, which means that satisfying requests for new connections may require changing the paths and/or the wavelength channels of already-configured connections. In a WDM cross-connect, disrupting connections in order to create new connections is undesirable since doing so requires buffering of the connections that are to be rearranged.
A WDM cross-connect is considered to be pathwise rearrangeably non-blocking in cases where connection requests can be routed through the cross-connect, but any additional requests received after routing the original set of requests may require some of the previously routed requests to be re-routed. Some WDM cross-connects are considered to be pathwise wide-sense non-blocking. These WDM cross-connects employ a routing algorithm that enables any sequence of connection requests and withdrawals to be satisfied without disturbing any of the currently routed requests. Pathwise strictly non-blocking cross-connects are known that enable any set of requests to be routed through the cross-connect without disturbing the routes associated with previous requests.
A request for a connection requires not only a route from the input fiber to the output fiber, but also a wavelength channel assignment along the route that only changes wavelength channels at wavelength interchangers and that begins and ends on the requested wavelength channels. These requests for connections between wavelength channels on input and output fibers are commonly referred to as demands. When a demand is made following a previously routed demand, the routes and/or the wavelength channel assignments associated with the previously routed demands may need to be changed. The definitions of wavelength rearrangeably non-blocking, wavelength wide-sense non-blocking and wavelength strictly non-blocking are analogous to the definitions provided above for pathwise rearrangeably non-blocking, pathwise wide-sense non-blocking and pathwise strictly non-blocking, respectively.
A WDM cross-connect that is both pathwise and wavelength strictly non-blocking will be referred to hereinafter as a strictly non-blocking WDM cross-connect. One known type of strictly non-blocking WDM cross-connect that it is capable of handling new requests for connections without disturbing those already existing utilizes k log k wavelength interchangers, where k corresponds to the number of input fibers and output fibers. Therefore, the number of wavelength interchangers utilized in this type of WDM cross-connect is relatively large. Since the overall cost of a WDM cross-connect is primarily attributable to the costs associated with the wavelength interchangers, it is desirable to minimize the number of wavelength interchangers incorporated into the WDM cross-connect. Therefore, it would be desirable to provide a strictly non-blocking cross-connect that minimizes the number of wavelength interchangers that are needed to provide the WDM cross-connect with strictly non-blocking properties.
FIG. 1 is a block diagram of a WDM cross-connect 1 that is commonly referred to as a standard design WDM cross-connect. The fabric 2 between the input optical fibers 3 and the output optical fibers 4 of the WDM cross-connect 1 includes a plurality of nodes (not shown) and a plurality of optical fibers (not shown) that interconnect the nodes. Each of the nodes is comprised of a wavelength granularity switch that switches signals received by the fabric 2 on the input fibers 3 onto selected output fibers 4. The WDM cross-connect 1 comprises a controller 6 that controls the operations of the fabric 2 and of the wavelength interchangers 5. The controller 6 causes the wavelength granularity switches to select an appropriate output fiber 4 so that the wavelength of the signal routed onto the output fiber 4 will not be the same as the wavelength of a signal that already exists on the output fiber 4.
The WDM cross-connect 1 comprises k wavelength interchangers 5, where k is a positive integer equal to the number of input fibers 3 and output fibers 4. Each wavelength interchanger 5 is connected to a single input fiber 3. Each input fiber 3 is capable of simultaneously carrying signals at n wavelengths, xcex1 through xcexn , where xcex denotes wavelength and n is a positive integer. Therefore, each input fiber supports n wavelength channels. Each of the wavelength interchangers 5 is capable of permuting the wavelength of a signal on the input fiber 3 to a different wavelength. The fabric 2 then causes the signal to be routed onto a selected output fiber 4. The controller 6 controls the selection of the wavelength channels by the wavelength interchangers 5.
FIG. 2 is a block diagram of a WDM cross-connect 7 that is commonly referred to as a modified standard design WDM cross-connect. The WDM cross-connect 7 is a modification of the design shown in FIG. 1 and includes a wavelength interchanger 8 connected to each of the output fibers 4. The WDM cross-connect 7 comprises 2 k wavelength interchangers. The additional wavelength interchangers 8 connected to the output optical fibers 4 enable the wavelength channel utilized by a signal on any of the output optical fibers 4 to be permuted. This enables demands to be handled that specify a particular output wavelength, which is not the case with the WDM cross-connect 1 shown in FIG. 1.
The additional wavelength interchangers 8 provide the WDM cross-connect 7 with improved versatility. However, 2 k wavelength interchangers are utilized by the WDM cross-connect 7, which significantly increases the cost of the cross-connect in comparison to the cost associated with the cross-connect shown in FIG. 1. The cross-connects 1 and 7 are, at best, rearrangeably non-blocking.
Accordingly, a need exists for a strictly non-blocking WDM cross-connect design that minimizes the number of wavelength interchangers that are needed to provide the WDM cross-connect with strictly non-blocking properties. A need also exists for such a WDM cross-connect that is suitable for use in a heterogeneous network, i.e., in a network that is comprised of sub-networks that are produced by different manufacturers. In accordance with the present invention, the number of input and output fibers of the WDM cross-connect may be different and/or the number of input and output wavelengths of the WDM cross-connect may be different. Furthermore, the input wavelengths may be different from the output wavelengths. Thus, in situations where downstream equipment or sub-systems require different wavelengths, a different number of wavelengths, a different number of fibers, a different number of wavelengths per fiber, etc., the WDM cross-connect of the present invention can be configured to meet those needs and to do so in a strictly non-blocking manner.
The present invention provides a strictly non-blocking WDM cross-connect that utilizes a relatively small number of wavelength interchangers. The present invention provides two embodiments for the strictly non-blocking WDM cross-connect, each of which is capable of utilizing different numbers of input and output optical fibers. The first embodiment of the WDM cross-connect of the present invention utilizes n1k1 wavelength interchangers whereas the second embodiment utilizes (k1+k2)xe2x88x921 wavelength interchangers, where k1, n1 and k2 are integers equal to the number of input optical fibers of the cross-connect, the number of wavelengths carried on each input optical fiber of the cross-connect, and the number of output optical fibers of the cross-connect, respectively. With respect to the first embodiment, n1k1 wavelength interchangers are used for situations where k2 is greater than k1. In situations where k1 is greater than or equal to k2, the WDM cross-connect in accordance with the first embodiment can be rendered strictly non-blocking by utilizing n2 k2 wavelength interchangers.
In accordance with the first embodiment, for the scenario where k2 is greater than k1, the WDM cross-connect comprises a single fabric, n1k1 wavelength interchangers and k1 optical switches. Each of the wavelength interchangers is connected to exactly one input port of the fabric and each input port of the fabric is connected to exactly one wavelength interchanger. The input optical fibers of the WDM cross-connect are connected to the k1 optical switches of the cross-connect, which separate out the n1 wavelengths onto n1 optical fibers, which are input to respective wavelength interchangers. Therefore, each wavelength interchanger receives exactly n1 optical fibers. The fabric has output ports that are connected to k2 respective output optical fibers. An analogous WDM cross-connect design can be configured for the scenario where k1 is greater than k2, but n2k2 wavelength interchangers and k2 optical switches will need to be used in combination with the fabric to render the cross-connect strictly non-blocking. Those skilled in the art will understand the manner in which this alternative configuration can be provided.
In accordance with another embodiment of the present invention, the WDM cross-connect comprises two fabrics and one or more wavelength interchangers that interconnect the fabrics. The cross-connect in accordance with this embodiment can be rendered strictly non-blocking by utilizing a number of wavelength interchangers equal to (k1+k2)xe2x88x921, where k1 and k2 correspond to the number of input and output optical fibers, respectively, of the WDM cross-connect.
In accordance with both of these embodiments, the number of input and output optical fibers of the strictly non-blocking WDM cross-connect can be unequal, although it is not required that they be unequal. This enables the WDM cross-connect of the present invention to be utilized with equipment located downstream of the WDM cross-connect that is configured to receive a different number of input optical fibers than the number of input optical fibers received by the WDM cross-connect.
These and other features and advantages of the present invention will become apparent to those skilled in the art from the following description, drawings and claims.